Gaseous discharge r. f. noise source



Oct. 7, 1958 l. sKo l 2,855,514

GASEOUS DISCHARGE R. F. NOISE SOURCE 2 Sheets-Sheet 1 Filed Dec. 20, 1955 INVENTOR. MERE/LL JKOL lK United States Patent GASEOUS DISCHARGER. F. NOISE SOURCE Merrill I. Skolnik, Watertown, Mass, assignor to the United States of America as represented by the $ecretary ofthe United StatesAir Force Application December 2i], 1955, Serial No. 554,380

1 Claim. (Cl. 250-36) It is the object of this invention to provide a source of continuous broad band radio frequency noise extending from very low frequencies to frequencies of the order of 4500 mc./s. or higher. It is a further object of the invention to provide a noise source capable of producing a substantial amount of noise power over this frequency band.

Basically, the noise source described herein is a coldcathode gaseous discharge relaxation circuit such as sometimes used for the generation of saw-tooth waveforms. Such a circuit comprises a source of direct current, a condenser connected through a charging circuit to the direct current source and a cold-cathode discharge device connected in shunt to the condenser to rapidly discharge the condenser when its voltage reaches the breakdown potential of the discharge device. Conventionally in such a circuit the output is taken from across the discharge device and has the form of a saw-tooth. In the described noise generator the output is taken across a resistor connected in series with the discharge device and therefore consists of a series of sharp pulses. The interval between pulses depends upon the power supply voltage, the size of the condenser, the resistance in the charging circuit, the size of the series resistor, the breakdown voltage of the discharge gap, the extinguishing voltage of the gas discharge, and the state of gas ionization prior to breakdown. The last parameter is not constant but fluctuates ina random manner, which results in a random variation of the pulse interval. This inherent aperiodic nature of the cold-cathode gaseous discharge relaxation circuit is important from the standpoint of noise generation since it is responsible for the continuous nature of the noise spectrum produced, as compared with the spectrum of discrete harmonically related frequencies that would result were the relaxations of the circuit periodic. The noise produced therefore is actually the Fourier spectrum of a series of very narrow pulses with a random repetition rate. The amount of noise produced is influenced by the gas and the cathode material. In general the gas should be electronegative because of the higher breakdown voltage and more rapid deionization of such gases, and the cathode material should be a poor thermionic emitter since any appreciable electron emission would result in a continuous arc and greatly reduced noise output.

The invention will be described in more detail in connection with the accompanying drawings, in which:

Fig. 1 is a schematic diagram of the noise generator;

Figs. 2 and 3 show methods of coupling the noise generator to an output circuit;

Figs. 4 and 5 show waveforms in the noise generator circuit, and

Fig. 6 is a graph of the noise spectrum produced by a noise generator in accordance with the invention.

Referring to Fig. 1 the noise generator is a relaxation circuit, the elements of which are a direct current power source having a voltage E; a current limiting resistor R; a condenser C; a cold-cathode gaseous discharge device 10 having a gas-filled envelope 11, an anode 12 and a cathode 13; and a series resistor r. The inherent capacity of the circuit is represented by the condenser 0 shown in dotted lines. The output circuit is connected across r. Suitable methods of coupling to the output circuit are shown in Figs. 2 and 3. In Fig. 2 a probe 14 connected to the center conductor of coaxial transmission line 15 is placed in proximity to the cathode lead of tube 10. In Fig. 3 an extension of the center conductor of line 15 is wrapped'about an insulating sleeve 16 surrounding the cathode lead.

The behavior of the above circuit is well known. Initially capacity C+c charges through resistor R, its Voltage E being applied between electrodes 12-13 of discharge device 10. When E becomes equal to the breakdown voltage of the gap in tube 10 the capacity is rapidly discharged through this tube and resistor r. When E has fallen tothe value at which the gaseous discharge in tube 10 cannot be maintained, the discharge circuit through r is-opened by the cessation of conduction in tube It) and the capacity C+c begins to charge again starting a new cycle of operation. The tube lil'therefore acts asaswitch serving to close and open the condenser discharge circuit through resistor r in response to the amplitude of the condenser voltage E The waveforms of E and l the current through r, are shown in Figs. 4 and 5, respectively. The period T between relaxations is given by the expression where E, is the initiating or breakdown voltage of the gas discharge and E is the voltage at which the discharge extinguishes. Taking a specific example, if E=1200 v., gas discharge and E is the voltage at which the discharge R=15,000 ohms, C+c=175 mmf, E =350 v. and E =20 v., then T=0.87 10' seconds. The discharge through r will not be oscillatory if T=R(C+c) log The corresponding potential across r is volts. For the smaller values of T the inherent capacity c of the circuit may be suificient.

The value of T is not constant but varies in a random manner from pulse to pulse. This is due mainly to the variation in breakdown potential that results from the natural random variation of the state of ionization of the gas in the discharge gap prior to breakdown. The noise produced in the output circuit is the Fourier spectrum of the series of pulses shown in Fig. 5. Due to the random variation of T this spectrum covers a very wide band and is continuous throughout the band. A typical noise spectrum obtained with a circuit of the type shown in Fig. l is given in Fig. 6. In this case the discharge gap consisted of A" aluminum electrodes separated 0.01". The gas was air at atmospheric pressure, and the average value of the current in resistor r was 50 ma. The noise factor plotted along the y-axis is the ratio of the produced noise power to the noise power available from a resistor The peak magnitude of the current pulse is at room temperature for the same bandwidth. The available noise power over a bandwidth B from a resistor at an absolute temperature T is P=kTB where k is the Boltzman constant. At room temperature (293 K.) the available noise power is 4 10- watts/me.

There are certain requirements as to electrode material and gas used in the discharge device. The anode may be made of any metal capable of withstanding the temperatures developed. The cathode, however, must be made of a metal having zero or negligible thermionic emission under the conditions encountered, since the presence of thermal emission from the cathode would result in a continuous arc and the absence of relaxation oscillations which are the principal source of noise. Aluminum and copper are suitable cathode materials. The gas used must be able to ionize and deionize in a time short compared to the relexation times of the circuit. Air at atmospheric pressure is a suitable gas and when used the envelope 11 is of course not required. Sulphur hexafluoride SP is another gas highly suited for this purpose.

The amount of noise produced depends principally upon the amplitude and duration of the pulses shown in Fig. 5. High pulse amplitude and short pulse duration yield increased noise. Gases with the highest breakdown voltage yield the greatest pulse amplitudes, whereas the width or duration of the pulse is determined either by the time constant of the condenser discharge circuit or by the build-up time of the gas discharge,

whichever is longer. The time constant of the discharge circuit is in most cases the controlling factor and therefore the product r(C-]-c) should be small for short pulse widths. The discharge gap is not critical but in general should be relatively small for high noise generations. Gaps lying within the range 0.005" to 0.15" have been found satisfactory.

I claim:

A radio frequency noise generator comprising a source of direct current; a condenser; a charging circuit including said condenser connected across said source; a discharging circuit comprising a resistance and a cold-cathode gaseous discharge device connected in shunt to said condenser; said gaseous discharge device comprising a gas impervious envelope containing an anode, a cathode, made of a metal producing negligible thermionic emission, and a gas having a high breakdown voltage and a high rate of ionization and deionization; and means for coupling a noise output circuit across said resistance.

References Cited in the file of this patent UNITED STATES PATENTS 2,235,667 Blount et al Mar. 18, 1941 2,284,101 Robins May 26, 1942 2,603,753 Axelsson et al July 15, 1952 FOREIGN PATENTS 134,942 Austria Oct. 10, 1933 

